For approximately how long is hydrofluoric acid applied to the intaglio surface of the FELDSPATHIC ceramic veneer before cementation?

Correct answer: D. About 60 seconds

• A. Fifteen to twenty seconds is the PHOSPHORIC acid etch time for enamel, and it is also close to the correct figure for LITHIUM DISILICATE, which is etched with 5% hydrofluoric acid for roughly 20 seconds. Reading which ceramic the stem specifies is what decides this item.
• B. Over-etching is counterproductive. Excessive dissolution of the glassy phase weakens the surface layer and REDUCES rather than increases bond strength, leaving a friable surface that fails cohesively.
• C. Application for minutes has no basis in any protocol, and hydrofluoric acid is highly caustic — it requires rubber dam or a try-in outside the mouth, and skin contact causes deep, delayed, painful burns.
• D. CORRECT. FELDSPATHIC and leucite-reinforced glass ceramics are etched with approximately 9.5% HYDROFLUORIC ACID for about 60 SECONDS. Hydrofluoric acid selectively dissolves the GLASSY MATRIX, exposing the crystalline phase and creating a micro-retentive porous surface. The etch is then thoroughly rinsed and a SILANE coupling agent applied — silane is what chemically links the silica in the ceramic to the resin, and etching without silaning loses most of the achievable bond strength.

Note: Hold both figures: 5% HF for about 20 seconds on LITHIUM DISILICATE, 9.5% HF for about 60 seconds on FELDSPATHIC and leucite-reinforced ceramic. ZIRCONIA is the exception to the whole protocol — it has no glassy phase, so hydrofluoric acid does nothing. Zirconia is air-abraded with alumina and treated with an MDP-containing primer instead.